The present invention relates to a method of manufacturing a multi-layers type solid-state image sensor obtained by stacking a photoconductive film on a substrate provided with solid-state image sensor.
A two-layered solid-state image sensor obtained by stacking a photoconductive film on a solid-state image sensor substrate has good characteristics of high sensitivity and low level of image smear. Thus, the solid-state image sensor of this type is promising for use in cameras of various types of TV monitors or high-definition TVs. An amorphous silicon film is often used as a photoconductive film for a solid-state image sensor of this type, and a silane glow discharge decomposition method is generally used as a method of depositing the amorphous film (e.g., refer to N. Harada et al. IEEE Trans. Electron Devices Vol. ED-32, No. 8 (1985) p. 1499).
However, a method of manufacturing a two-layered image sensor by the amorphous silicon film deposition method using the glow discharge decomposition method has the following problems. More specifically, since this method uses plasma for decomposing a source gas, many charged particles are present in a gas phase and damage the obtained film, thus degrading the characteristics of the film. As a result, in a solid-state image sensor which uses an amorphous silicon film as a photoelectric conversion layer, the image lag characteristics resulting from the amorphous silicon film are degraded due to the degradation in the quality of the amorphous silicon film or the interface characteristics.
Assume that a structure wherein a p-type amorphous silicon carbon film is provided as a barrier layer on an i-type amorphous silicon film, i.e., a p-type a-SiC/i-type a-Si structure is adopted as the structure of the photoconductive film having an amorphous silicon film. When such a structure is formed by the glow discharge decomposition method, the interface characteristics between the films of the composite film comprising the p-type a-SiC/i-type a-Si structure are degraded. As a result, recombination of electrons and holes generated by light incidence tends to occur at the interface, and the sensitivity in a blue region which is absorbed in the vicinity of the surface is decreased.
In order to solve such a problem of interface characteristics, a method to include a graded region, wherein the carbon content is continuously changed, in the a-SiC/a-Si interface is also proposed. However, this method makes the process control difficult and eventually degrades the image-lag characteristics.